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New Scientist Live

West Nile virus transmitted in breast milk

By Emma Young

The discovery of West Nile virus antibodies in a one-month-old baby makes it “very likely” that the virus can be transmitted through breast milk, says the US Centers for Disease Control.

Samples of milk from the mother contained both the virus and antibodies. “The infant had minimal outdoor exposure and exposure to mosquitoes, so we think it is very likely that the child became infected via the breast milk,” Lyle Peterson of the CDC said on Thursday. “But we can’t determine this with 100 per cent certainty.”

Previously, traces of West Nile virus had been identified in breast milk, but the risk of transmission to babies had been theoretical.

The CDC also reported a sharp rise in the death toll from the virus, reaching 136 on Thursday, up from 125 just 24 hours before. A total of 2631 people have been infected across the continent so far in 2002. Up to 15 of the cases are thought to have been the result of receiving transfusions of infected blood.

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Warm and wet

West Nile virus is transmitted through the bite of infected mosquitoes, which thrive in warm, wet conditions. The coming of autumn does not lessen the threat, say entomologists at Purdue University, Indiana. Depending on the weather, October can be among the worst months for West Nile infection, they say.

The CDC recommends precautions against mosquito bites, such as wearing clothing and using repellents. It says mothers should not switch to formula milk, given the relatively low West Nile transmission risk and the well-documented benefits of breast-feeding.

Only one in five people infected with the virus develop symptoms, and only about 20 per cent of these people develop potentially fatal brain inflammation. The one-month-old baby is showing no symptoms.

West Nile virus was first identified in Uganda in 1937. The disease has spread rapidly from coast-to-coast in the US since the first human case in the continent was reported in New York just three years ago.